This invention relates to a variable displacement oil pump for an internal combustion engine for a vehicle.
In recent years, there is an demand that an oil discharged from an oil pump has two stepped (stepwise) characteristics which are a low pressure characteristic in a first engine speed region, and a high pressure characteristic in a second engine speed region, for using the oil discharged from the oil pump, in, for example, devices such as sliding portions of an engine, a variable valve actuating apparatus arranged to control operation characteristics of engine valves, which have different desired discharge pressures.
A Japanese Patent Application Publication No. 2008-524500 (corresponding to U.S. Patent Application Publication No. 2009/022612 A1, U.S. Patent Application Publication No. 2010/329912 A1, U.S. Patent Application Publication No. 2013/098446 A1, and U.S. Patent Application Publication No. 2013/195705 A1) discloses a variable displacement pump devised to satisfy the above-described demands. The above-described variable displacement pump includes a cam ring whose an eccentric amount with respect to a rotor is varied by moving against a spring urging force of a spring member; and two pressure receiving chambers which confront each other, and which are formed on an outer circumference surface of the cam ring. In this variable displacement pump, the pump discharge pressure is selectively acted to these pressure receiving chambers, so that the cam ring is actuated in two stepped (stepwise) manner.